Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled to the stimulation lead(s). Thus, electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of neural tissue. In particular, electrical energy conveyed between at least one cathodic electrode and at least one anodic electrodes creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neurons beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing current pulses (anodes) and which of the electrodes are sinking current pulses (cathodes) at any given time, as well as the magnitude and duration of the current pulses. The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. In the case of SCS, such a therapeutic benefit is “paresthesia,” i.e., a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
To produce the feeling of paresthesia without inducing discomfort or involuntary motor movements within the patient, it is often desirable to preferentially stimulate nerve fibers in the dorsal column (DC nerve fibers), which primarily include sensory nerve fibers, over nerve fibers in the dorsal roots (DR nerve fibers), which include both sensory nerve fibers and motor reflex nerve fibers, as well as those involved with proprioception.
While DC nerve fibers are the intended targets in conventional SCS, in fact, the DR nerve fibers often are recruited first because of geometric, electric, anatomical, and physiological reasons. For example, the DR nerve fibers include fibers with larger diameters than the largest nearby DC nerve fibers, and thus, have a lower threshold at which they are excited. Other factors that contribute to the lower threshold of the DR fibers include their potentially preferential orientation over that of the DC fibers, and the inhomogeneity and anisotropy of the surrounding medium at the entrance of the DR nerve fibers into the spinal cord. Thus, DR nerve fibers may still generate APs at extracellular stimulation levels than will nearby DC nerve fibers. As a result, the DC nerve fibers that are desired to be stimulated can have a lower probability of stimulation than do the DR nerve fibers, and thus, the proprioceptive fibers can often be recruited leading to patient discomfort. Also, the motor reflex arc can be activated leading to undesired motor recruitment. Both cases can prevent a paresthesia concordant with the patient's pain area.
For reasons such as these, it is often desirable to preferentially stimulate DC nerve fibers over the DR nerve fibers by modifying the threshold at which neural tissue is activated in a manner that maximizes excitation of the DC nerve fibers, while minimizing excitation of the DR nerve fibers; that is, to increase the DR/DC threshold ratio. This can be accomplished by sinking an electrical pulse to a cathodic electrode located at the center of the spinal cord to depolarize the target tissue adjacent the cathodic electrode, thereby creating APs along the DC nerve fibers, while an electrical pulse can be sourced to anodic electrodes on both sides of the cathodic electrode to hyperpolarize non-target tissue adjacent the anodic electrodes, thereby increasing the threshold of the DR nerve fibers.
In a typical example, in order to stimulate the DC nerve fibers, while guarding against the stimulation of the DR nerve fibers, SCS systems may activate anodes that flank a single cathode in a medial-lateral electrical field, with the single cathode providing the stimulation energy for the DC fibers, while the flanking anodes increase the thresholds, and therefore, guarding against the recruitment, of the DR fibers, as illustrated in FIG. 1.
Because several DR nerve fibers on each side of the spinal cord may be close enough to the cathode to be inadvertently stimulated, several anodes on each side of the cathode may be activated in order to guard against stimulation of these DR nerve fibers. For example, in one approach, a center cathode provides the stimulation energy for the DC fibers, and two pairs of anodes (one pair rostro-caudally flanking the cathode on one transverse side of the spinal cord, and the other pair rostro-caudally flanking the cathode on the other side of the spinal cord) guard against stimulation of the DR nerve fibers, as shown in FIG. 2. Typically, the current is distributed to the anodes equally (i.e., 25% current on each anodes) under the assumption that all loci of dorsal root fibers are equally affected by the anodes.
This assumption may be proper when the DR fibers perpendicularly enter the spinal cord. However, as illustrated in FIG. 3, the angle between the DR nerve fibers and the longitudinal axis of the spinal cord tends to increase as one moves down the spinal cord in the caudal direction (i.e., the DR nerve fibers located at the more rostral vertebral segments will enter the spinal cord at a more shallow angle (perhaps even perpendicularly), whereas the DR nerve fibers located at the more caudal vertebral segments will enter the spinal cord at a steeper angle. In the latter case, an equal current distribution on the anodes illustrated in FIG. 2 may not be optimum.
There, thus, remains a need for an improved technique for preferentially stimulating DC nerve fibers over DR nerve fibers.